1. Field of the Invention
The present invention relates to fields of molecular and developmental biology. More particularly, the present invention relates to microRNAs that regulate differentiation, proliferation and death of cardican and skeletal muscle cells.
2. Description of Related Art
Cellular differentiation and organogenesis involve restricted zones of transcriptional regulation that govern gene expression patterns during specific temporal windows. One mechanism for regulating the target genes activated by transcriptional regulators involves the dose-sensitive response of cis elements to gradients of DNA-binding proteins. In this scenario, variances in the levels of transcription factors result in the activation or repression of diverse target genes allowing finer control of the spatial and temporal events of organogenesis.
MicroRNAs (miRNAs) mediate a recently recognized form of translational inhibition that alters dosages of critical regulators and thereby provides a mechanism for temporo-spatial control of developmental and homeostatic events in a wide range of plant and animal life (He and Hannon, 2004; Ambros, 2004; Meister and Tuschl, 20040. Genetic studies in Caenorhabditis elegans and Drosophila melanogaster suggest important functions for specific miRNAs in cell death and proliferation decisions through direct interaction of miRNAs with target sequences in messenger RNAs (Lee et al., 1993; Wightman and Ruvkun, 1993; Moss et al., 1997; Brennecke et al., 2003; Abrahante et al., 2003; Johnston and Hobert, 2003; Vella et al., 2004; Chang et al., 2004). However, an understanding of the specific roles and regulatory pathways controlled by mammalian miRNAs has been limited by the lack of reliable and specific methods to identify miRNA targets.
The transcriptional regulation of cardiomyocyte differentiation and cardiogenesis is highly conserved and requires sequential activation or repression of genetic programs (Chien and Olson, 2002; Srivastava and Olson, 2000). Early during heart formation cardiomyocytes proliferate even as they begin to differentiate, however they soon exit the cell cycle as differentiation progresses. Serum response factor (SRF) binds to CArG boxes in the regulatory region of numerous muscle-specific and growth-regulated genes and thus has a dual role in regulating the balance between proliferation and differentiation during cardiogenesis, in part through interaction with tissue-specific co-factors (Norman et al., 1988; Miralles et al., 2003; Shin et al., 2002; Chen et al 2002). Failure to maintain an adequate pool of undifferentiated myocyte precursors could result in organ hypoplasia as is observed in zebrafish that lack the transcription factor, Hand2 (Yelon et al., 2000). In this case, the ventricular pool of cardiomyocytes is greatly diminished, similar to the defect in ventricular expansion observed in mice lacking Hand2 along with its relative, Hand1 (Srivastava et al., 1995; Srivastava et al., 1997; Firulli et al., 1998; Yamagishi et al., 2001; McFadden et al., 2005). While dynamic temporal and spatial expression of regulatory pathways is important in cardiogenesis, whether microRNAs are involved in refining cardiac transcriptional activity is unknown.